Sync EN: add xlink namespace and intro description to UrlValidationErrorType (#498)

Syncs reference/uri/uri.whatwg.urlvalidationerrortype.xml with php/doc-en up
to 8352249a, covering:
- #5474: intro simpara description (translated)
- #5481: xmlns:xlink declaration on <reference>

Closes #496

Author: lacatoire

reference/uri/uri.whatwg.urlvalidationerrortype.xml

@@ -1,13 +1,14 @@
 <?xml version="1.0" encoding="utf-8"?>
-<!-- EN-Revision: 330903cf6a567b885d24d0a468960d1e3b9ae213 Maintainer: lacatoire Status: ready -->
-<reference xml:id="enum.uri-whatwg-urlvalidationerrortype" role="enum" xmlns="http://docbook.org/ns/docbook">
+<!-- EN-Revision: 8352249a84d0b91b38c741ff1c256061127dbfbb Maintainer: lacatoire Status: ready -->
+<reference xml:id="enum.uri-whatwg-urlvalidationerrortype" role="enum" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns="http://docbook.org/ns/docbook">
  <title>La enumeración Uri\WhatWg\UrlValidationErrorType</title>
  <titleabbrev>Uri\WhatWg\UrlValidationErrorType</titleabbrev>
 
  <partintro>
   <section xml:id="enum.uri-whatwg-urlvalidationerrortype.intro">
    &reftitle.intro;
    <simpara>
+    Los posibles errores de validación definidos en el <link xlink:href="&url.url.whatwg-url;">estándar WHATWG URL</link>.
    </simpara>
   </section>